Four Point ProbeProcedure for Pro4 using Keithley

Overview What is Four Point Probing How the system works Pro 4 Set Up Simple Calculations behind Four Point

Probing Procedure for using Pro4

What is Four Point Probing

Four Point Probing is a method for measuring the resistivity of a substance.

Impurity concentrations can be estimated from the resistivity

Resistivity vs Sheet Resistance

Bulk or volume resistivity (r) is measured in ohms-cm

Independent of sample size or shape

Sheet resistance (rs) is measured in ohms-per-square

Can be used to measure the value a resistor in a IC

Pro-4 Set Up

Pro-4 probing station from LUCAS LABS with 4 point probe head

KEITHLEY 2400 power/source meter

Computer with Pro4 software and interface

The 4 point probing setup consists of 3 key components

Probing Station

Source Meter

Pro-4 Software

The 4 point probing setup can measure resistivity or the thickness of a film. But, either one has to be known.

Resistivity Probe Stand

Contact Lever

Probe head electrical connection

Probe Head

Mounting Chuck (Aluminum base

with Teflon surface

How the system works Current is passed through the two outer probes Voltage is measured between the two inner

probes Read and record both current and voltage

values from the Keithley source meter Sheet Resistance is measured using (V/I) and k V = volts, I = Amps (convert current reading to

amps) k=constant factor = to 4.53 when the wafer

diameter is much greater than the probe spacing – typical for wafers

Sheet resistance (rs) = (k)(V/I)= ohms/square

For the bulk resistivity of a wafer

The thickness of the wafer/film must be known – use calipers to measure the wafer thickness

Convert caliper reading in mm to cm

Resistivity of wafer r=rs x thickness in cm

There is a second k factor but for our work this k factor is not a factor and can be ignored (typically >.995)

Sample Wafer Calculations A current of 1.0 mA is passed

through the wafer and a voltage reading of 0.030 v is noted. I = 1.0 MA = .001 amp

V/I = .030 v/.001A = 30 ohm rs = (V/I) k = (.030/.001)(4.53) =

(30)(4.53) = 135.9 ohms/square The wafer is measured as 0.40 mm

= .04 cm r = (135.9)(.04cm) = 5.43 ohm-cm

The Pro-4 can be used to measure resistivity or the thickness. But, either one has to be known.

The # of points to be tested and the shape of the sample can be selected.

A single point or multiple points on the sample can be tested to obtain the average resistivity

Resistivity measurements

1. Click on the Auto Test tab for multiple measurements or Single Test tab for a single reading.

2. Select size and shape of the sample using the tabs at the bottom of the page. Type in the required information.

3. Place the sample on the mounting table and then move the sample to position it at the required location

4. Turn down the lever so that all the needles on the probe head are in contact with the wafer.

Procedure for using the Pro-4

After the measurement is completed, the resistivity at each location will be displayed on the left hand side of the screen.

When all the points are tested, the data can be saved and read using excel

Saving the data